Overview
Suggestions
Copyright © 1998
Nova Scotia Museum
Six Suggestions
Total magnification of a compound microscope is the power of the objective multiplied by the power of the eyepiece (i.e. 10X eyepiece with a 6X objective = 60X magnification). Although many student microscopes boast of high magnification, they are extremely difficult to use in the 600X - 1200X range. Focusing and locating the specimen become tedious. Light and image quality are usually poor. It is also difficult to obtain specimens thin enough for high power magnification. Don't be oversold on high magnification. A maximum of 300X - 500X is fine for most student uses.
Very high power objectives (100X) require a drop of oil between the lens and the slide. Although often touted as a selling point, these oil immersion lenses are difficult to use and, in children's microscopes, do not deliver a worthwhile image.
Zoom
Some microscopes have a zoom lens component that allows continuously
variable magnification in a certain range, something like a telephoto camera
lens. So instead of a 10X eyepiece and choice of 6X, 10X or 30X objectives,
you can "zoom" the magnification of the eyepiece from 10 to 20 times, giving
continuous increase from 60X to 120X if the 6X objective were selected,
for example. Generally, it is not possible to retain the same image quality
with zoom lenses as you expect from fixed lenses. The few zoom systems
which have recently appeared for research microscopes are complex and expensive.
Light
Most compound microscopes require light to pass upward through the
transparent specimen to your eye. A light source may be built into the
microscope beneath the stage. If the light is battery-operated, you may
find it a source of expense and frustration because the power cells have
a very short life span. We have also noticed many problems with poor electrical
connections on popular children's microscopes.
Most instruments are equipped with a mirror that allows you to reflect
an outside light source up through the specimen. A bright desk lamp or
sunny window makes a good light source, and is more reliable than the cheap
built-in kind. Try for a bright, evenly lit field of view. Shine your light
on the mirror, not on the specimen.
Some microscopes have a condenser between the mirror and the stage.
This may be a movable disc with holes of different sizes in it, or a diaphragm
you can open and close. Reducing the size of the condenser opening can
give a sharper image if the light is bright.
Lenses
Check that the eyepiece lens is held firmly in its mount and is not
scratched. Generally, the larger the diameter of the eyepiece lens, the
easier it will be to use. Objective lenses should be fixed in sturdy, metal
mounts. Shorter, or low-power objectives are most useful. Check that the
turret holding the objectives turns smoothly, with click stops for each
lens. Glass lenses are preferable to plastic. Some kits offer two eyepieces
of different magnification, say a 10X and a 15X. Read the section on magnification
before you spend extra money for these.
Mechanical parts
The success of your microscope depends greatly on how sturdy it is.
The eyepiece and objective are separated by a body tube. Either this tube
or the stage must move up and down to allow you to focus on each specimen.
This movement is accomplished with a rack-and-pinion mechanism. It should
be smooth and fairly firm so that it does not slip.
Most microscopes have a pivot between the base and the body tube. Be
sure this is firm: once you have set the microscope at a comfortable angle,
it should not tilt further.
The base should be heavy enough and broad enough to hold the instrument
without tipping. Check the rest of the instrument for flaws in the finish.
Metal is more durable than the plastics commonly used.
Accessories
You may be offered everything from shrimp eggs to butterfly wings to
tweezers to stains. Some tools are useful: tweezers, a probe (needle on
a stick) and an eye dropper. As for bee legs and butterfly wings, just
collect your own.
Cell stains are informative when properly used, but your child will
need to do some reading or obtain advice to enjoy them. Canada Balsam is
used for making permanent slides, which is something a student will not
likely want to do, because slides and cover slips are the necessary items
microscope kits rarely provide in adequate quantity.
Don't pay for accessories you can collect or make yourself, but do try
to acquire lots of blank slides and cover slips.
Overview | Suggestions | Working With Your Microscope